1. Field of the Invention
The present invention relates to reflective guest-host-liquid-crystal display devices, and in particular, to techniques for improving efficiency of the use of incident light by using a quarter-wavelength-plate layer and a light-reflection layer, both built into a reflective guest-host-liquid-crystal display device. In more particular, the present invention relates to the structure of a black matrix for shielding light in the peripheries of pixels.
2. Description of the Related Art
A reflective guest-host-liquid-crystal display device including a quarter-wavelength-plate and a light-reflection layer is disclosed in, for example, Japanese Unexamined Patent Publication No. 6-222351, and its section view is shown in FIG. 4. The reflective guest-host-liquid-crystal display device 101 includes a pair of upper and lower substrate 102 and 103, guest-host liquid crystal 104, a dichroic dye 105, a pair of upper and lower transparent electrodes 106 and 110, a pair of upper and lower alignment layers 107 and 111, a light-reflection layer 108, and a quarter-wavelength-plate layer 109. The pair of substrates 102 and 103 is composed of insulating material such as glass, quartz or plastic. At least, the upper substrate 102 is transparent. The guest-host liquid crystal 104, which includes the dichroic dye 105, is held between the pair of substrates 102 and 103. The guest-host liquid crystal 104 includes nematic liquid crystal molecules 104a. The dichroic dye 105 is so-called xe2x80x9cp-type dyexe2x80x9d having transition dipole moments almost parallel to the major axes of its molecules. On the inner surface 102a of the upper substrate 102 are formed switching devices (not shown). The transparent substrates 106 are patterned in a matrix to form pixel electrodes, which are driven by the corresponding switching devices. The outer surface of the upper substrate 102 is coated with the alignment layer 107, which is composed of polyimide resin. The surface of the alignment layer 107 is processed, for example, by rubbing, and horizontally aligns the nematic liquid crystal molecules 104a. 
In addition, the light-reflection layer 108, which is composed of aluminum, and the quarter-wavelength-plate layer 109, which is composed of high-molecular liquid crystal, are formed in the order given on the surface 103a of the lower substrate 103. The transparent electrodes 110 and the alignment layer 111 are formed in the order given on the quarter-wavelength-plate layer 109.
Subsequently, the operation of the reflective guest-host-liquid-crystal display device 101 when performing monochrome display will be briefly described.
When no voltage is applied, the nematic liquid crystal molecules 104a align horizontally, and the dichroic dye 105 aligns similarly. When light incident from the upper substrate 102 travels into the guest-host liquid crystal 104, components of the incident light having an oscillating plane parallel to the major axes of the molecules of the dichroic dye 105 are absorbed by the dichroic dye 105. Other components having an oscillating plane vertical to the major axes of the molecules of the dichroic dye 105 pass through the guest-host liquid crystal 104, and are circularly polarized by the quarter-wavelength-plate layer 109 formed over the inner surface 103a of the lower substrate 103. The circularly polarized components are reflected by the light-reflection layer 108. At this time, the reflected light is polarized in the reverse direction, and passes through the quarter-wavelength-plate layer 109 again to form components having a polarizing plane parallel to the major axes of the molecules of the dichroic dye 105. The formed components are absorbed in the dichroic dye 105, which generates almost completely black display.
In addition, when a voltage is applied, the nematic liquid crystal molecules 104a align vertically along the direction of the electric field, and the dichroic dye 105 aligns similarly. Light incident from the upper substrate 104 passes through the guest-host liquid crystal 104 without being absorbed in the dichroic dye 105, and is reflected by the light-reflection layer 108 without being substantially affected by the quarter-wavelength-plate layer 109. The reflected light passes through the quarter-wavelength-plate layer 109 again to be emergent without being absorbed in the guest-host liquid crystal 104, which generates white display.
According to the conventional structure shown in FIG. 4, the switching devices for driving the pixels are formed on the emergent-side substrate. The switching devices consist of thin film transistors. The switching devices shield the incident light, which reduces the aperture ratio of the pixels by the amount of the shielding. To cope with this point, there has been developed a structure in which switching devices are formed on the reflection-side substrate, whose example is shown in FIG. 5. As shown in FIG. 5, an upper substrate 201 has a counter electrode 203a totally formed thereon, which consists of a transparent electrode, and a lower substrate 202 has pixel electrodes 204a consisting of a matrix of segmented reflector electrodes. In other words, this example is an active matrix type. On the outer surface of the lower substrate 202, the pixel electrodes 204a, which are patterned in a matrix, and the thin film transistors (TFTs) corresponding thereto are formed. The TFTs are used as switching devices for respectively driving the pixel electrodes 204a. In other words, by selectively switching the TFTs, a signal voltage can be written into the corresponding TFTs. The drain region D of each TFT is connected to the pixel electrode 204a. The source region S thereof is connected to a signal interconnection 221. The gate electrode G thereof is connected to a gate interconnection. An auxiliary capacitor Cs is formed to correspond to each pixel electrode 204a. The pixel electrode 204a is electrically separated by a planarizing layer 222 from the TFT, the auxiliary capacitor Cs and the signal interconnection 221. On the outer surface of the upper substrate 201 is totally formed the counter electrode 203a. An electro-optical material 205 is held between both substrates 201 and 202, which are opposed, with a predetermined gap provided therebetween. The electro-optical material 205 has a layered structure including guest-host liquid crystal 206 and a quarter-wavelength-plate layer 207. The guest-host liquid crystal 206 contains nematic liquid crystal molecules 209 and dichroic dye 208, and are horizontally aligned by upper and lower alignment layers 210 and 211. The quarter-wavelength-plate layer 207 is formed along the pixel electrode 204a. 
Writing the signal voltage to the pixel electrode 204a generates an electric field between it and the opposing counter electrode 203a, which changes the guest-host liquid crystal 206 between its absorbing condition and its transparent condition. This optical change is generated for each pixel electrode, which enables the displaying of the desired image. Below the pixel electrode 204a are positioned the TFT, the auxiliary capacitor Cs and the signal interconnection 221. There are not these component parts in the incident optical path, which does not affect the pixel aperture ratio. In other words, the area of the pixel electrode 204a can be totally used as the pixel aperture, which enables bright display.
The above-described reflective guest-host-liquid-crystal display device uses exterior light like natural light without using a backlight for back lighting. Thus, in order to obtain the bright image, the aperture ratio of pixels needs to be increased. Concerning this point, the structure shown in FIG. 5 enables a sufficient pixel aperture ratio, as described above. In addition, when color display is performed in the structure shown in FIG. 5, three-primary-color filters corresponding to each pixel is provided on the counter substrate 201. Improving the contrast of the color display requires provision of a black light-shield (black matrix) along the border of each pixel. A conventional black matrix is provided on a counter substrate, similar to the color filter. This structure requires the precise alignment of the black matrix-formed counter substrate 201 and the TFT-formed substrate 202. The mechanical precision has a limit, which generates, to some extent, an error in the alignment. In order to absorb this error, the width of the black matrix is designed slightly broad. The aperture ratio deteriorates by the amount of the broadening.
Accordingly, it is an object of the present invention to provide a reflective guest-host-liquid-crystal display device for solving the foregoing problem in which a quarter-wavelength-plate layer and a light-reflection layer are included so that incident light is effectively used.
To this end, the foregoing object has been achieved through provision of a reflective guest-host-liquid-crystal display device including: first and second substrates jointed to each other, with a predetermined gap provided therebetween; and guest-host liquid crystal including a dichroic dye, the guest-host liquid crystal held in the gap, in which the first substrate includes: switching devices; a light-reflection layer; a quarter-wavelength-plate layer, formed above the switching devices and the light-reflection layer, having a contact hole in conduction with the switching devices; pixel electrodes, formed by patterning the surface of the quarter-wavelength-plate layer, connected to the switching devices by the contact hole; and a light-shielding black matrix formed along the border of each pixel electrode, and the second substrate has a counter electrode formed on the outer surface thereof.
Preferably, the light-reflection layer consists of a light-shielding resin film having irregularities formed on one surface; and a reflective metal film on the light-shielding resin film, corresponding to the pixel electrodes, and the black matrix is included in the light-shielding resin film.
The light-shielding resin film may be a photosensitive resin film having a black pigment or dye added therein.
The gap between the light-reflection layer and the quarter-wavelength-plate layer may be filled with a planarizing layer.
The planarizing layer may be colored so as to correspond to each pixel electrode, whereby functioning as a color filter.
According to the present invention, a so-called xe2x80x9con-chipxe2x80x9d structure is employed. This enables the precise alignment of a black matrix and color filters with pixel electrodes, which can provide a sufficient pixel aperture ratio. The on-chip structure eliminates the need of the precise alignment of a counter electrode-formed substrate and a pixel electrode-formed substrate. Accordingly, the black matrix does not need to have a margin of error.